Do you guys ever recommend a little dip or “trap” for the bathroom exhaust duct when its in the attic? I see them all the time. Ive talked to a couple contractors who say they do it cause here in Alberta it gets ridiculously cold so its really hard to not get some condensation running back down the vent. But every once in awhile i see a bathroom vent in the attic without one. Obviously im only referring to vents with longer runs. Usually cause of the height of the attic space. Sorry for the crappy photo but i figured it might help everyone understand.
that would not be an issue for me…
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No. A dip would collect water which, in our weather, would freeze rendering the exhaust fan useless.
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I’m in Minnesota just south of the border near Thunder Bay. I work in a cold climate, with construction/GC experience.
An air trap like your image is an accepted method to restrict air backflow, although some say it is not acceptable for makeup combustion air - a different application. However, if the fan is not used regularly, that moisture may freeze in the trap. Yes, yours is insulated, a good component. If it’s commonly used in your area, it probably has a history of working. There are other cold climate exhaust duct methods.
What is done here is to use aluminum duct that is buried deep in the insulation. The duct is run to the gable and is under insulation the whole run. The hood located at the gable end has a flap. This system tests well sealed with blower door testing.
You need a duct flap somewhere. Otherwise you have air leakage through the fan ducts, even if the fan is off. In-line flaps are available. Panasonic fans, used here a lot, have a flap in the unit itself.
Turn on all the exhaust fans in the house plus the dryer. Keep this one off. Put the back of your hand near the fan grill and check for cold air. Use a smoke stick, check the air temperature with a temperature probe if you have one. An IR camera is perfect if you have one.
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When I built, I kept them well insulated reducing condensation. And, I recommended the same on reports, as needed.
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I agree with others that it’s not something I have ever called out or even looked for. IMO the important thing is that the vent terminates to the exterior to prevent transferring moisture from a steamy bathroom into the attic space. And as Larry mentioned, insulation around the duct is a good idea as well.
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I think I’d be more worried about what kind of muck would collect and grow in a “vent trap”. A better fix is probably a timer switch to keep the fan running longer and fully exhausting the moisture from showers etc.
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Wouldn’t it evaporate in time? The damper would still stop very cold air from entering the vent. Im hearing alot of different opinions on this.
Did you see a flap or shutter at the termination point, as your post suggests? If so, that’s good.
“Evaporate over time”: Likely, in summer, but not before black and white mold/fungus like biological growth develops.
You should talk to the installers and ask them about their long term experience with this configuration. It probably works but is not the best configuration IMO. The success is dependent on exhaust air temperature not being reduced to the point of condensation forming inside the pipe. The pipe under the insulation method doesn’t have that issue until it is exhausted to the outside air. I probably would mention it and definitely would test to see if it has a flap or shutter in the system.
I came across a ceiling vent yesterday in a second floor bathroom with no flap or shutter and reported that issue.
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Never. No water traps.
The attic should be as cold as the outside in winter, so…
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But not the inside of the insulated vent. Which is where the condensation would accumulate when the hot and humid air meets the cold at the top.
Thats a really great response, i appreciate your input.
The cold is all around, as per your picture, and the insulation will only delay the inevitable.
Recalling a conversation with a HVAC contractor from several decades ago; he reported fixing a bath fan duct system that was installed on top of the ceiling insulation. It leaked condensation/bulk water onto the ceiling in winter. When the fan was off, warm moist air cooled, the dew point was reached, water changed from gas to liquid and ran back in the pipe onto the ceiling. Your trap is preventing the above from happening by trapping liquid water in the trap.
That picture looks ok. I think that he was asking if one should be present.
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Ahh, gotchya. I think you are right.
No, don’t put a trap in the exhaust duct. If you are getting a significant amount of condensation, something is broken or not installed right.
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Instead of running that much duct uphill terminating at roof, they should have taken it out the side wall, running downhill toward the exterior, that would take care of any condensation issues. That’s how they’re doing it in my area and have for several years. I’m sure it would work just as well in colder climates.
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I asked myself the same thing.
The duct was installed on an angle and has a slight bow. No issues.